Jason
The piece missing from the helpful advice above is that for best results a NAS should be running a media server like Twonky. Many NASes have this or something similar installed already, though it's usually possible to install after the fact. Do make sure though that your NAS is a 'proper' one actually running an operating system though, rather than do what I did and buy a 'newtwork drive' which is that and only that.
I'd also recommend the (admittedly outrageously priced) n-Stream iPhone app for the Uniti, which allows you to remote control all of its functions, including negotiating the music library, and makes the Uniti much more user-friendly.
Lastly, if you're starting from scratch I'd recommend ripping to FLAC since you don't have to worry about finding a media server that will transcode on the fly.
Further long-term musings on the Uniti here.
cheers
JD